Environmental concerns related to the nondegradable, petroleum-based plastics used in the production of packaging and disposable consumer goods has fostered research in the area of biodegradable materials for packaging and other articles of manufacture. Several biodegradable, biological polymers have been investigated because of their film-forming capabilities. Three major types of biodegradable products are derived from starch, from fermentation of sugars to polyesters, i.e., poly-(3-hydroxybutyrate)-co-(3-hydroxyvalerate), and from synthetic lactic acid polymers.
For example, starch-based resins have been converted into compost bags, disposable food-service items (e.g., cutlery, plates, and cups), packaging materials (e.g., loosefill and films), coatings, and other speciality items. However, a major disadvantage of the starch-based materials is moisture sensitivity due to the hygroscopic nature of starch.
Inexpensive, renewable, and abundant plant proteins constitute a viable source of biodegradable polymers. Accordingly, plant protein-based biodegradable polymers have been studied as potential biodegradable materials for packaging and other disposable consumer goods. For example, a transparent sheet having acceptable strength and biodegradability has been prepared from wheat gluten. Starch mixed with zein has been used in molding applications, in part because the biodegradability of zein is about equal to that of starch.
Corn zein, therefore, has been investigated for use as a structural material in packaging applications to take advantage of the unique properties of zein. For example, corn zein has been used as a base material to produce biodegradable sheets.
Zein is the prolamine in corn, and is an abundant protein in corn gluten meal, which is a coproduct of corn wet milling. Corn gluten meal contains about 70% protein (dry base) with zein amounting to 60% of that protein.
Zein is located in small round particles, 1-2 μm in diameter, called protein bodies in maize endosperm. Three distinct fractions, α, β, and Υ zein, have been identified by differential solubility in aqueous alcohol solutions. Native zein is a mixture of proteins that differ in molecular size, solubility, and charge.
Zein is separated from the corn gluten meal by solvent extraction, usually with isopropanol. The extract is clarified centrifugally, then chilled to precipitate the zein. Additional extractions and precipitations increase zein purity. The zein then is dried to a powder.
Approximately 80% of native zein is soluble in 95% ethanol, with the remainder soluble in 60% ethanol. These two fractions are designated α-zein and β-zein, respectively. β-Zein consists of disulfide-crosslinked α-zein. The reducing environment of the steeping process in corn wet milling cleaves disulfide bonds, leaving essentially only α-zein in commercial corn gluten and zein. α-Zein is a polypeptide having a molecular weight about 21,000 to about 25,000.
Zein is a unique material in terms of thermoplasticity and hydrophobicity. When zein is heated with starch above 60° C., the mixture becomes a dough and exhibits viscoelastic behavior. The moldable properties of zein classify zein as a plastic, i.e., a polymeric material capable of flowing under applied pressure or heat. The hydrophobic nature of zein is related to its high content of non-polar amino acids. Because of this hydrophobic property, zein has been used in the food and pharmaceutical industries as coating material for candies, rice, dried fruits, and nuts. Zein also has been used in the pharmaceutical industry to coat capsules for protection, controlled release, and masking of flavors and aromas. Zein possesses the additional benefits of being renewable and biodegradable.
Zein also has been investigated for use as a structural material in packaging applications because of its film-forming properties. In particular, zein forms tough, glossy, scuff-proff, grease-proof films or coatings that are resistant to microbial attack. Zein also has an ability to cure with formaldehyde to provide an essentially inert product. A zein coating, therefore, can function as an oxygen, lipid, and moisture barrier.
Zein-based coatings have been used in numerous applications, for example, the extend the similarly, zein-based coatings have protected confectioneries, dried fruits, and dried vegetables from moisture absorption, oxidation, and/or lipid migration. Zein coatings also have been used to coat vitamin-enriched rice, thereby reducing or eliminating vitamin loss. A zein-based coating also was used to maintain a high concentration of sorbic acid at the surface of an intermediate-moisture food, and to reduce moisture and firmness loss and delay a color change (i.e., reduce oxygen and carbon dioxide transmission) in fresh tomatoes.
A typical method of producing a zein-based film or coating involves solubilizing the zein in an aqueous alcohol solution, then casting the zein-containing alcohol solution on an inert, flat surface. After the water and alcohol evaporate, a tasteless and glossy film remains. The film then is peeled from the flat surface and used for its intended purpose. Typical disadvantages of zein films formed by casting are the control of film thickness and homogeneity in properties.
Cast films thus formed are very tough and resistant to grease. However, the cast films also are brittle and require the addition of plasticizers to impart flexibility and reduce the possibility of film chipping, cracking, and shattering. Commonly used plasticizers present in zein films include, for example, glycerin, triethylene glycol, fatty acids (like oleic, stearic, and palmitic acid), glycol monoesters, glyceryl monoesters, acetylated monoglycerides, dibutyl tartrate, lactic acid, and tricresyl phosphate.
An alternative method of preparing zein films involves plasticization of zein by forming an emulsion with oleic acid, followed by precipitation of the zein-oleic acid mixture to form a soft moldable resin. The dough-like resin then is stretched over rigid frames to obtain thin membranes that set into flexible films, that are ductile and tough. Microstructure images of the films showed a high degree of structural development consisting of fibers and ribbon-like protein structures that are theorized as responsible for the flexibility and toughness of the films.
Another method of preparing zein-based films utilizes corn zein plasticized with palmitic or stearic acid. Using these ingredients, a moldable zein resin was prepared, then cold-rolled into sheets about 0.5 mm thick. Such cold-rolled sheets are too thick for use in many practical applications. Typically, zein films are preferred for practical applications. As used herein, a “film” is defined as having a thickness of about 250 μm (microns) or less. A “sheet” is defined as having a thickness greater than about 250 μm.
Zein films exhibit relatively good water vapor barrier properties compared to other biodegradable, edible films, but are inferior compared to low density polyethylene (i.e., LDPE) and hydrolyzed ethylene-vinyl acetate (i.e., EVOH). Zein films also have a water vapor permeability (WVP) about equal to or lower than films made from cellophane, methylcellulose, ethylcellulose, ethylcellulosepolyvinylpyrrolidone, hydroxypropylmethylcellulose, and hydroxypropylcellulose, under similar test conditions. The WVP of zein films increases with increasing plasticizer level and relative humidity surrounding the film. Zein sheets plasticized with palmitic or stearic acids provide sturdy films having low water absorption compared to films made from other biological polymers.
The following publications are directed to zein, plasticized zein, and zein resins, and to methods of preparing films from such materials:
G. W. Padua, “Biodegradable plastics from corn proteins,” Biobased products Expo '04, Kansas City, Mo. (1994).
G. W. Padua, “Biodegradable Resins from Corn By-Products,” Presentation to AOSCA 6th Annual Identity preserved Crops Conference, Chicago, Ill. (1995).
M. Masco-Arriola, “Preparation and Evaluation of Biodegradable Plastics Derived from Corn Zein,” M. S. Thesis University of Illinois, (1996).
H. M. Lai, “Preparation of Zein-Based Biodegradable Materials and the Investigation of their Physical Properties,” Ph. D. Thesis University of Illinois (1997).
H. M. Lai et al., Cereal Chem., 74, pp. 83-90 (1997).
H. M. Lai et al., Cereal Chem., 74, pp. 771-775 (1997).
H. M. Lai et al., Cereal Chem., 74, pp. 194-199 (1997).
H. M. Lai et al., J. Appl. Polymer Sci., 71, pp. 1267-1281 (1999).
G. W. Padua et al., “Properties of biodegradable plastics derived from corn proteins,” Proceedings from the Third Biomass conference of the Americas, Montreal, Canada, Aug. 24-29, 1997. Ed. R. P. Overend and E. Chronet Elsevier Science, Tarrytown, N.Y. (1997).
H. M. Lai et al., “Development of corn zein-based biodegradable resins,” Book of Abstracts, Institute of Food Technologists Annual Meeting, New Orleans, La. (1996).
H. M. Lai et al., “Development of corn zein-based biodegradable sheets,” Abstracts from the Fifteenth Midwest Food Processing Conference, LaCrosse, Wis. (1996).
H. M. Lai et al., “Effect of processing method on water barrier properties of zein-based films,” Book of Abstracts, Institute of food Technologists Annual Meeting, Orlando, Fla. (1997).
F. X. B. Santosa et al., “Effect of fatty acid content on tensile properties of zein-based biodegradable resin sheets,” Book of Abstracts, Institute of food Technologists Annual Meeting, Orlando, Fla. (1997).
T. Ha et al., “Extrusion processing of zein-based biodegradable plastics,” Abstracts from the Sixteenth Annual Midwest Food Processing Conference, IFT Regional Conference, LaCrosse, Wis. (1997).
F. X. B. Santosa et al., “Tensile and water absorption properties of zein-fatty acid biodegradable resins,” Abstracts from the Sixteenth Annual Midwest Food Processing Conference, IFT Regional Conference, LaCrosse, Wis. (1997).
T. Ha et al., “Extrusion processing of zein-based biodegradable plastics,” Book of Abstracts, Institute of Food Technologists Annual Meeting, Atlanta, Ga. (1998).
F. X. B. Santosa, “Thermal behavior of zein sheets plasticized with oleic and linoleic acids,” Book of Abstracts, Institute of Food Technologists Annual Meeting, Atlanta, Ga. (1998).
H. M. Lai et al., “Structure characterization of biodegradable zein resin films by x-ray diffraction,” Book of Abstracts, Institute of Food Technologists Annual Meeting, Atlanta, Ga. (1998).
M. T. Izzo et al., Cereal Chem., 66, pp. 47-51 (1989).
J. L. Kanig et al., J. Pharm. Sci., 51(1), p. 77 (1962).
J. W. Park et al., J. Food Sci., 59(4), pp. 916-919 (1994).
The above publications show that there is a long-standing need for biodegradable articles of manufacture, like food packaging materials and disposable consumer goods. Such articles of manufacture typically are prepared from sheets or films of a resin. Although abundant biodegradable materials, like zein, are available for such articles of manufacture, no practical method of preparing commercial quantities of zein-based articles presently exists. The present-day methods of preparing zein based sheets and films, like casting, are not amenable to providing articles of manufacture due to practical limitations, for example, an inability to form uniform sheets of a desired, predetermined thickness, or an inability to form a commercial quantity of the film in a practical time frame. The present invention is directed to a method of preparing biodegradable sheets, films, and formed products from a zein-based resin, and thereby overcoming the disadvantages of prior casting and cold-rolling methods. The films, sheets, and formed products can be made in commercial quantities and in practical time frames using the present method of manufacture.